This invention relates to the art of materials science and, more particularly, to nonmetallic materials such as ceramic and intermetallic materials. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Ceramic materials have certain outstanding properties, such as high temperature strength, resistance to thermal shock, fatigue resistance, corrosion resistance, low density, and low thermal expansion, which make them attractive materials for high temperature applications. For instance, if ceramics can be used as engine components, an engine could be run at a higher temperature, and thus be much more efficient than it could be with metal components. This has sparked a great deal of interest in studying various ceramics for these types of applications.
Ceramics differ from metals in one very important aspect; they do not show any yield upon loading. The lack of a stress-relieving characteristic, which gives ceramics their brittle nature and low tolerance for flaws, is a major drawback to using them in structural applications. One of the methods of dealing with this mechanical characteristic, which can be referred to as lack of fracture toughness, is the development and use of ceramic composites, which are ceramics having another material, usually a fiber, dispersed throughout them. Mechanisms which may act when ceramics are made tougher by addition of fibers of a different material include the transfer of load from the matrix to the fiber based on elastic modulus, microcracking or prestressing due to a difference in thermal expansion, crack deflection, and phase transformation toughening.
However, the problem is only partially solved; ceramic composites are still subject to brittle failure. There is a class of materials which offers the advantages of a ceramic and certain of the beneficial mechanical characteristics of a metal. These materials are intermetallics, which at high temperature have the excellent properties of a ceramic, but mechanically behave more like a metal, since they show yielding and stress-relieving characteristics.
Molybdenum disilicide (MoSi.sub.2) is an intermetallic compound which has potential for structural use in oxidizing environments above 1200.degree. C. It possesses a melting point of 2030.degree. C. and its oxidation resistance at high temperature is very good. Mechanically, MoSi.sub.2 behaves as a metal at high temperatures, since it undergoes a brittle-to-ductile transition at approximately 1000.degree. C. Thus, MoSi.sub.2 has a stress-relieving characteristic at high temperatures. The major problems impeding the use of MoSi.sub.2 as a high temperature structural material (potential use temperatures in the range of 1200.degree.-1800.degree. C.) are its relatively low strength at high temperatures and its brittleness at low temperatures. At low temperatures, strength is limited by brittle fracture, while at high temperatures, it is limited by plastic deformation or creep. For this material to be a viable structural material at high temperatures, both its elevated temperature strength and its room temperature fracture toughness must be improved. Fracture toughness may be defined as resistance to fracture.
Silicon carbide whiskers made by a vapor-liquid-solid (VLS) process have been used to reinforce MoSi.sub.2. This use resulted in improved ambient temperature fracture toughness and a near doubling of strength at 1200.degree. C. (compared to room temperature strength). However, the improvement in strength is not enough. The present invention is the use of whiskers of a different size made by a different process, a vapor-solid (VS) process, as a reinforcing material.
The two MoSi.sub.2 composites which are discussed herein, the inventive composite and the prior art composite, are reinforced with 20 vol % of either VLS beta-SiC whiskers made at Los Alamos National Laboratory or VS beta-SiC whiskers purchased from J. M. Huber Corporation of Borger, Tex. and designated Huber XPW.sub.2 whiskers. SiC whiskers are minute, high-purity, single crystal fibers. They have very high stiffness in the longitudinal direction, in which they are grown. The main difference between the two whisker types used is their size. In hot-pressed shapes of VLS reinforced MoSi.sub.2, the VLS whiskers were about 100 to 200 microns long, about 3 to 15 microns in diameter, and had an aspect ratio ranging from about 20:1 to about 30:1. VS whiskers in hot-pressed shapes were from about 1 to about 5 microns in length, had a diameter of from about 0.1 to about 0.5 micron, and had an aspect ratio of from about 5:1 to about 15:1. A few VS whiskers were 100 to 200 microns long in the as-purchased condition, but were broken down to less than 5 microns long during dry blending with MoSi.sub.2 powder.